Transmission of television signals in a digital format has been viewed as promising for a number of years. Digital systems providing teleconference quality video have become commonplace in both government and industry. However, digital transmission of toll-grade or broadcast quality television signals has not yet achieved such acceptance.
This results, in part, from the broadcasters, reluctance to allow any kind of processing on the transmitted signals. To a greater extent, digital transmission of broadcast quality video has failed to gain acceptance because it has not been cost-effective. The lack of available wideband digital links, as well as the complexity of implementation of bandwidth efficient digital video CODEC (coder/decoder) has kept the cost of digital television transmission too high to compete with analog methods.
Advances in very large-scale integration, as well as recent work in the field of advanced digital modulation techniques, have combined to make digital video processing technically feasible and potentially cost competitive for broadcast quality television transmission. The coupling of a transparent, bandwidth efficient, data compression technique with a bandwidth efficient modulation technique offer the potential for a transmission of two or more high-quality television signals in the same bandwidth occupied by a signal frequency-modulated television signal.
In the past, differential pulse code modulation (DPCM) has been one of the most popular predictive image coding methods of video signals due to its simplicity of implementation and overall subjective performance characteristics. One of the most serious problems with DPCM schemes has been that three to four bits/pixel were required to achieve acceptable image quality, with four bits/pixel generally preferred to maintain a broadcast quality picture representation.
Patents which appear to be relevant to the invention described herein are as follows:
U.S. Pat. No. 4,125,861 to Mounts et al describes a method and apparatus for decreasing the entropy of an encoded signal by 25% over conventional techniques which employ DPCM. Mounts et al utilize a DPCM predictor, a non-uniform adaptive quantizer, and a variable length encoder for data compression of video images. The adaptive quantizer, depending on picture content, adaptively forces the quantizer output to a particular value different from the normal output. This forced change places more quantized picture elements into particular quantization levels, thus taking greater advantage of the compression gained by the variable length encoder. The forced change of quantizer output level is acceptable only when it is not harmful to the picture fidelity.
U.S. Pat. No. 4,396,906 to Weaver describes a method and apparatus for implementation of a Huffman encoder/decoder which utilizes a particular code word structure to simplify the encode/decode process. The code word structure is a "truncated Huffman code set" which allows the encoding and decoding circuitry to be greatly simplified over the circuitry required for conventional Huffman code sets. One drawback of using the "truncated Huffman code set" is that the set is not optimal and will not provide as much compression as an optimal Huffman code set.
U.S. Pat. No. 4,363,036 to Subramaniam describes a method for compressing digital data, which method is useful in facsimile transmission. The technique is not applicable to encoding of NTSC television images due to the specific nature of the scanned facsimile data. A document for facsimile transmission is scanned to generate a digital image for encoding and subsequent transmission. Each pixel is either white or black and is represented by a "one" or a "zero", respectively. A non-adaptive predictive technique is used to predict the pixel values and source states for each pixel. The prediction Table and Source State Tables are pregenerated based upon the Markov model of several source images.
U.S. Pat. No. 4,667,251 to Hasegawa describes a method and apparatus useful for the encoding and transmission of half-tone images. A dithering process is used to convert an analog half-tone image into a binary code. Typically, the binarized picture signal contains a large number of white-to-black transitions which, therefore, does lend itself to efficient encoding for transmission. According to this invention, the analog halftone signal is binarized by a dithering process and then is passed through a correlation processing stage prior to encoding for transmission.
U.S. Pat. No. 4,494,108 to Langdon et al discloses a method for adaptively modeled symbol source statistics to achieve efficient compression coding. An encoder adaptively computes and maintains statistics on the input data and uses the statistics to encode the data into a variable length string via a linearized tree structure. The decompression circuitry detects the ends of the variable length codes and decodes them. The data is then reconstructed using an adaptive statistics unit and a model structure unit.